1. Field of the Invention
The present invention relates to an apparatus for driving light emitting elements such as LEDs at high voltage, and also, to an electronic appliance employing such an apparatus.
2. Description of the Related Art
FIG. 1 shows an apparatus for driving light emitting elements disclosed in Japanese Unexamined Patent Application Publication No. 2003-332624. The apparatus shown in FIG. 1 includes arrays of light emitting elements 121 to 126, constant-current drivers 112, 113, and 114 to which first ends of the arrays of light emitting elements are connected, respectively, a selector 118 to select a lowest one from among voltages applied to the constant-current drivers 112, 113, and 114, and a power source circuit 127 to compare the selected voltage from the selector 118 with a reference voltage, provide an amplified differential signal according to the comparison result, and regulate an output voltage to be applied to second ends of the rows of light emitting elements.
The constant-current drivers 112, 113, and 114 employ active elements such as transistors or MOSFETs to pass constant currents through the light emitting elements 121 to 126. The active elements each have three terminals, i.e., an emitter, a collector, and a base, or a gate, a drain, and a source.
In a case where the active element is a transistor, a current passing through the collector and emitter thereof is controllable with a current passing through the base and emitter thereof. In another case where the active element is a MOSFET, a current passing through the drain and source thereof is controllable with a voltage applied between the gate and source thereof. The transistor has a characteristic that, even if the base current is unchanged, the collector current changes if a voltage applied between the collector and emitter of the transistor changes. The MOSFET has a characteristic that, even if the gate voltage is unchanged, the drain current changes if a voltage applied between the drain and source of the MOSFET changes.
The transistor differently operates in a saturation region and an active region. In the saturation region, the collector current changes depending on the collector-emitter voltage irrespective of a control signal applied to the control terminal (base terminal). In the active region, the collector current changes depending on the control signal irrespective of the collector-emitter voltage. Similarly, the MOSFET differently operates in a linear region and an active region. In the linear region, the drain current changes depending on the drain-source voltage irrespective of a control signal applied to the control terminal (gate terminal). In the active region, the drain current changes depending on the control signal irrespective of the drain-source voltage.
The apparatus for driving light emitting elements according to the related art includes a predetermined margin in the reference voltage so that the constant-current drive elements such as transistors or MOSFETs may always operate in the active region, and on that basis, the related art makes the lowest voltage selected by the selector 118 agree with the reference voltage.